A complimentary metal-oxide semiconductor (“CMOS”) image sensor includes an array of pixels that generate electrical signals in response to incident light. Ideally, the electrical signal generated by a pixel accurately represents the amount of light incident on that pixel. In practice, however, even when the pixel is not being exposed to light, it still generates some electrical signal. Such signal, generally referred to as the “dark current,” is undesired because it reduces the dynamic range of the image sensor.
Since it is extremely difficult to eliminate dark current completely, black level correction (“BLC”) methods have been developed to effectively mitigate the error that dark current may introduce into an imaging pixel. Some BLC methods utilize pixels in the pixel array that are physically shielded from light by a metal layer or other shield disposed over the pixels. These designated pixels are typically referred to as the optically black pixels (hereinafter, “dark calibration pixels”) and are usually clustered in rows or columns immediately adjacent to an imaging region of unshielded pixels (hereinafter, “imaging pixels”).
Conventional BLC methods are best suited for the condition when the rate of dark current generation is the same for both the dark calibration pixels and the imaging pixels. The dark calibration pixels and the imaging pixels generally have the same offset error introduced by various shared circuit elements in readout circuitry (e.g. amplifiers and analog-to-digital converters (“ADC”)). Therefore, when the dark calibration pixels and the imaging pixels have the same dark current, one BLC method is to simply subtract the dark calibration pixel readout from the imaging pixel readout. Because the dark calibration pixel readout includes both the offset error from readout circuitry and the dark current error, subtracting the dark calibration readout from the imaging pixel readout yields an imaging pixel value correcting for offset and dark current error.
However, this BLC method is not suitable for the case when the rate of dark current generation is different (non-uniform) for the dark calibration pixels and the imaging pixels because the dark current component of the dark calibration pixel readout will not match the imaging pixel. Therefore, simply subtracting the dark calibration pixel readout from the imaging pixel readout will yield inaccurate corrected imaging pixel values. In practice, the dark calibration pixels may have additional factors that contributed to dark currents that are substantially different from dark currents of the imaging pixels. One of the factors may be mechanical stresses due to the physical shielding layer over them. These mechanical stresses on the dark calibration pixels may lead to higher or lower dark currents compared to the imaging pixel dark currents. The non-uniformity in dark currents between the dark calibration pixels and the imaging pixels makes a BLC method that takes into account the non-uniformity of dark currents and the circuitry offset desirable.